Magnetic tapes have found various applications in audio tapes, videotapes, computer tapes, etc. In particular, in the field of magnetic tapes for data-backup (or backup tapes), tapes having memory capacities of 100 GB or more per reel are commercialized in association with increased capacities of hard discs for back-up. Therefore, it is inevitable to increase the capacity of this type of tape for data-backup. It is also necessary to increase the feeding speed of tape and a relative speed between the tape and heads in order to quicken the access speed and the transfer speed.
To increase the capacity of tape for data-backup per one reel, it is necessary to increase the length of tape per reel by decreasing the total thickness of the tape, to reduce the thickness demagnetization so as to shorten the recording wavelength by forming a magnetic layer with a thickness as very thin as 0.3 μm or less, and to increase the recording density in the tape-widthwise direction by narrowing the widths of the recording tracks.
When the thickness of the magnetic layer is reduced to 0.3 μm or less, the durability of the tape tend to lower. Therefore, at least one primer layer is provided between a non-magnetic support and the magnetic layer. When the recording wavelength is shortened, the influence of spacing between the magnetic layer and the magnetic heads becomes serious. Thus, if the magnetic layer has large projections or dents, the output decreases due to the spacing loss, so that the error rate increases.
When the recording density in the tape-widthwise direction is increased by narrowing the width of the recording tracks, magnetic flux leaking from the magnetic tape is decreased. Therefore, it is needed to use, for reproducing heads, MR heads which make use of magneto-resistance elements capable of achieving high output even when the magnetic fluxes are very small.
Examples of the magnetic recording media which can correspond to MR heads are disclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In these magnetic recording media, skewness of output from the MR heads is prevented by controlling the magnetic fluxes from the magnetic recording medium (a product of a residual magnetic flux density and the thickness of the medium) to a specific value or less, or the thermal asperity of the MR heads is reduced by lessening the dents on the surface of the magnetic layer to a specific value or less.
When the width of the recording tracks is decreased, the reproduction output lowers due to off-track. To avoid such a problem, track servo is needed. As types of such track servo, there are an optical servo type (JP-A-11-213384, JP-A-11-339254 and JP-A-2000-293836) and a magnetic servo type. In either of these types, it is necessary that track servo is performed on a magnetic tape which is drawn out from a magnetic tape cartridge (which may be also called a cassette tape) of single reel type which houses only one reel for winding the magnetic tape, in a box-shaped casing body. The reason for using a single reel type cartridge is that a tape cannot be stably run in a two-reel type cartridge which has two reels for drawing out the tape and for winding the same, when the tape-running speed is increased (for example, 2.5 m/second or higher). The two-reel type cartridge has other problems in that the dimensions of the cartridge become larger and that the memory capacity per volume becomes smaller.
As mentioned above, there are two types of track servo systems, i.e., the magnetic servo type and the optical servo type. In the former track servo type, servo bands as shown in FIG. 10 are formed on a magnetic layer by magnetic recording, and servo tracking is performed by magnetically reading such servo bands. In the latter optical servo type, servo bands each consisting of an array of pits are formed on a backcoat layer by laser irradiation or the like, and servo tracking is performed by optically reading such servo bands. Other than these types, there is such magnetic serve type in which magnetic servo signals are also recorded on a magnetized backcoat layer (for example, JP-A-11-126327). Further, in other optical servo type, optical servo signals are recorded on a backcoat layer, using a material capable of absorbing light or the like (for example, JP-A-11-126328).
Then, the principle of the track servo system is simply described by way of the former magnetic servo type.
As shown in FIG. 10, in the magnetic tape (3) for the magnetic servo type, servo bands (200) for track serve which extend along the lengthwise direction of the tape and are spaced from one another at about 2.8 mm intervals, and data tracks (300) for recording data thereon are formed on the magnetic layer. Each servo band (200) consists of a plurality of servo signal-recording sections (201) on which the respective servo track numbers are magnetically recorded. A magnetic head array (not shown), which records and reproduces data on a magnetic tape, consists of a pair of MR heads for servo track (forward running and backward running) at both ends, and for example, 8×1 pairs of recording-reproducing heads (in which the recording heads are magnetic induction type heads and the reproducing heads are MR heads) which are arranged between both the MR heads for servo tracking. In response to a signal from the MR head for servo track which has read a servo signal, the entire magnetic head array moves interlocking with each other, so that the recording-reproducing heads move in the widthwise direction of the tape to reach the data tracks (for example, in case of the magnetic head array on which 8×1 pairs of recording-reproducing heads are arranged, eight data tracks are formed corresponding to a pair of serve tracks).
In this stage, the magnetic tape runs in such a state that one of both tape edges extending along the lengthwise direction is regulated in its tape widthwise position by the inner surface of a flange of a guide roller provided in a magnetically recording-reproducing unit (a tape-driving unit) (see FIG. 8). As seen in FIG. 4, the edge (3a) of the magnetic tape (3) generally has corrugated unevenness called edge weave or edge wave (unevenness formed by the waving of the widthwise edge of the tape alongside the tape lengthwise direction). Therefore, the magnetic tape (3), even though running alongside the inner surface of the flange as the reference for the tape running, very slightly fluctuates in its position in the widthwise direction. However, this problem is solved by employing the above-mentioned servo system: that is, even if the position of the magnetic tape very slightly fluctuates in the widthwise direction, the entire magnetic head array moves in the tape widthwise direction in association with such a fluctuation, so that the recording-reproducing head can always reach the correct data tracks.
In this case, if the tape has a high edge weave α having a frequency [(V/f): s−1=Hz] of 80 Hz or more (particularly 200 Hz or more), wherein V is a tape-running speed and f is a cycle of the edge weave, the magnetic head array cannot follow the tracks. Thus, dislocation from the tracks occurs. However, such dislocation from the tracks is not so serious, in case where the recording track width is as wide as 30 μm or more, and where the difference between the recording track width and the reproducing track width [(recording track width)−(reproducing track width)] exceeds 16 μm (for example, the recording track width is about 80 μm, and the reproducing track width, about 50 μm). This is because, when the recording track width is as wide as 30 μm or more and when the above difference exceeds 16 μm, the recording track width is sufficiently wider than the reproducing track width, so that the magnetic head array can run on the recording tracks, even if dislocation of several μm from the track occurs. Thus, such dislocation does not leads to a decrease in output.
In another case, when a temperature or a humidity changes, dislocation from the tracks tends to occur, because the magnetic tape expands or contracts in the tape-widthwise direction in association with such a change. The methods of solving this problem by lessening the coefficients of temperature and/or humidity expansion of a magnetic tape are disclosed in JP-A-04-106723, JP-A-09-219016, JP-A-04-106723, JP-A-11-096545 and JP-A-11-250449. However, such dislocation from tracks in association with a change in temperature and/or humidity is not so serious for the same reason as above, in case where the recording track width is as wide as 30 μm or more, and where the above difference exceeds 16 μm. In this regard, although the expansion of a magnetic tape in the lengthwise direction due to a change in temperature and/or humidity may change the recording wavelength or the like, correction with circuits is possible for such a change.
As a result of further investigation, it is found that such a change in temperature and/or humidity does not induce a serious problem under specified conditions, even though the recording track width is 30 μm or less, and the above difference is 16 μm or less. That is, a decrease in reproduction output due to off-track hardly causes a problem under the following conditions (a) or (b): (a) dislocation from the tracks due to a change in temperature and/or humidity is small, although dislocation from the tracks due to the edge weave is large; and (b) dislocation from the tracks due to the edge weave is small, although dislocation from the tracks due to a change in temperature and/or humidity is large.
However, as a result of more intensive investigation, it was revealed that a decrease in reproduction output due to off-track tends to occur, even though the amount of the edge weave and the coefficient of thermal and/or humidity expansion are each singly negligibly small, in case where the recording track width is so narrow as 28 μm or less and the above difference of [(recording track width)−(reproducing track width)] is 16 μm or less. While there is a fluctuation of several micrometers in position between the recording heads and the reproducing heads in the apparatus, this fluctuation becomes at least two times larger under the worst combination of the conditions. The dislocation from the tracks due to the edge weave and a change in temperature/humidity further gives adverse influence, which results in a decrease in reproduction output. This phenomenon is remarkable when the difference of [(recording track width)−(reproducing track width)] is 10 μm or less.
When the width of the recording track is further reduced to 21 μm or less, a decrease in reproduction output due to off-track appears in spite of about 2 μm of edge weave, which has raised no problem in the conventional recording tracks. This is because, when the reproducing track width should be equal to a conventional one in order to ensure a reproduction output, the off-track margin becomes narrower. Further, when the recording track width is as narrow as 21 μm or less, it is confirmed that not only the absolute value of edge weave but also the cycle of the edge weave and the tape running speed have a complicated relationship with the off-track. To apply the servo system to a magnetic tape having recording tracks with a width as narrow as 21 μm or less, a relationship among the cycle and the amount of edge weave, the recording track width, the reproducing track width and the tape running speed with respect to the head followability is carefully examined. As a result, the following are revealed: a position error signal (or PES, i.e. a value indicating a fluctuation in positional dislocation; the value of a standard deviation 1) becomes higher, resulting in a tracking error, if the values of [α/(Tw−Tr)] and [α/(Tw−Tr)]×(V/f) exceed specific values, wherein α is an amount of the edge weave (in the tape widthwise direction of the tape edge (the direction Y–Y′ on FIG. 4)) having a cycle of f; V [mm/second] is a tape running speed; Tw [μm] is a recording track width; and Tr [μm] is a reproducing track width. This problem is considered to arise as follows. Since the magnetic head array as a whole has large mass, the magnetic head array cannot move following the motion of the magnetic tape in the widthwise direction, when the values of [α/(Tw−Tr)] and [α/(Tw−Tr)×(V/f)] exceed specific values. As a result, a position error signal or PES becomes higher. In case where the off-track margin is small, the off-track becomes larger to cause a such a tracking error.